The La-5's heritage began even before the outbreak of war, with the LaGG-1, a promising yet underpowered aircraft – turning a full circle, for example, took 20 seconds. The LaGG-3 was a modification of that design that attempted to correct this by both lightening the airframe and fitting a more powerful engine. Nevertheless, this was not enough, and the lack of power remained a significant problem.

In early 1942, two of the LaGG-1 and -3's designers, Semyon Lavochkin and Vladimir Gorbunov, attempted to correct this deficiency by experimentally fitting a LaGG-3 with the more powerful Shvetsov ASh-82 radial engine. Since the LaGG-3 was powered by an inline engine, they accomplished this by grafting on the nose section of a Sukhoi Su-2 (which used this engine). By now, the shortcomings of the LaGG-3 had caused Lavochkin to fall out of Joseph Stalin's favor, and factories previously assigned to LaGG-3 construction had been turned over to building the rival Yakovlev Yak-1 and Yak-7. The design work required to adapt the LaGG-3 to the new engine and still maintain the aircraft's balance was undertaken by Lavochkin in a small hut beside an airfield over the winter of 1941-1942, all completely unofficially.

When the prototype took flight in March, the result was extremely pleasing - the fighter finally had a power plant that allowed it to perform as well in the air as it had been supposed to on paper. After flying, the LaG-5 (the change in name reflecting that one of the original LaGG designers was no longer with the programme), Air Force test pilots declared it superior to the Yak-7, and intensive flight tests began in April. After only a few weeks, the design was modified further, cutting down the rear fuselage to give the pilot better visibility.

By July, Stalin ordered maximum-rate production of the aircraft, now simply known as the La-5 and the conversion of any incomplete LaGG-3 airframes to the new configuration. The prototype was put in mass production almost immediately in factories located in Moscow and in the Yaroslav region. While still inferior to the best German fighters at high altitudes, the La-5 proved to be every bit their match closer to the ground. With most of the air combat over the Eastern Front taking place at altitudes of under 5,000 m (16,404 ft), the La-5 was very much in its element. Its rate of roll was excellent.

Further refinement of the aircraft involved a fuel-injected engine, further lightening of the aircraft, and fixed slats to improve all-round performance. This was designated the La-5FN and would become the definitive version of the aircraft. A full circle turn took 18–19 seconds. Altogether, 9,920 La-5's of all variants were built, including a number of dedicated trainer versions, designated La-5UTI. Several La-5's had three Berezin B-20 cannon installed in the nose capable of a salvo of 3.4 kg/s rounds. Further refinements of the aircraft would lead to the Lavochkin La-7.